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Re: [Qemu-devel] Some question about savem/qcow2 incremental snapshot

From: Eric Blake
Subject: Re: [Qemu-devel] Some question about savem/qcow2 incremental snapshot
Date: Tue, 8 May 2018 09:41:40 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0

On 12/25/2017 01:33 AM, He Junyan wrote:
hi all:

I am now focusing on snapshot optimization for Intel NVDimm kind
memory. Different from the normal memory, the NVDimm may be 128G, 256G
or even more for just one guest, and its speed is slower than the
normal memory. So sometimes it may take several minutes to complete
just one snapshot saving. Even with compression enabled, the snapshot
point may consume more than 30G disk space.
We decide to add incremental kind snapshot saving to resolve this. Just
store difference between snapshot points to save time and disk space.
But the current snapshot/save_vm framework seems not to support this.
We need to add snapshot dependency and extra operations when we LOAD
and DELETE the snapshot point.
Is that possible to modify the savevm framework and add some
incremental snapshot support to QCOW2 format?

In general, the list has tended to focus on external snapshots rather than internal; where persistent bitmaps have been the proposed mechanism for tracking incremental differences between snapshots. But yes, it is certainly feasible that patches to improve internal snapshots to take advantage of incremental relationships may prove useful. You will need to document all enhancements to the qcow2 file format and get that approved first, as interoperability demands that others reading the same spec would be able to interpret the image you create that is utilizing an internal snapshot with an incremental diff.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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